This invention relates to television transmitters and receivers. More specifically, this invention relates to enhanced transmission and reception of HDTV signals.
The bandwidth of video signals from high definition TV cameras is substantially larger than the bandwidth of conventional (NTSC) signals. In order to fit HDTV transmission in the spectrum normally allocated to conventional TV channels and concurrently permit reception by conventional TV receivers, a substantial reduction in the bandwidth of HDTV signals is necessary, in addition to innovative allocation of the spectrum. In order to allow for terrestrial HDTV transmission concurrently with terrestrial NTSC transmission, one proposal is that conventional TV transmissions remain unaltered and that the HDTV transmissions occupy the spectrum that is currently devoted to separating the NTSC channels. These separation channels are normally referred to as the "taboo" channels.
Two noise interference concerns must be addressed, however. One is the interference of the HDTV signals in the NTSC broadcasts, and the other is the interference of the NTSC signals in the HDTV broadcasts. Before these are addressed however, it may be useful to first describe the proposed HDTV format.
In the proposed HDTV format, each frame consists of analog signals and digital signals. The timing is controlled so that the analog signals are transmitted during the frame signal portion of adjacent NTSC channels, and the digital signals are transmitted during the retrace portion of the frame signals of adjacent NTSC channels. This approach takes advantage of the fact that noise introduced by HDTV signals during a retrace interval is not displayed and is therefore harmless to the NTSC broadcasts. Consequently, a less stringent bandwidth limiting of the HDTV digital signal is permissible. The analog portion of the HDTV digital signal comprises a collection of concatenated PAM pulses that describe the energy in an error signal which is derived by comparing the frame's video signal to the immediately previous frame's video signal that is motion compensated.
Noise introduced by the NTSC channels has the greatest potential for disruption of the received image when it corrupts the digital signals of HDTV. However, since during the retrace interval the NTSC signals are quiescent, the problem is not severe; particularly since the digital signals can employ error detection and correction techniques. To minimize the effects of noise on the analog signal, the aforementioned application discloses a number of complementing solutions. One is combining PAM pulses, another is decomposing large signals into smaller ones or appending such large signals to the digital signal (coupled with gain control of the analog signal), still another is introducing signal leak, and lastly, employing scrambling.
The concept of combining PAM pulses improves noise immunity by converting a group of adjacent low amplitude pulses into a single PAM pulse of larger amplitude. Whereas low amplitude PAM pulses can be greatly affected by additive noise, the combined PAM pulses (when properly encoded) affects only one of the constituent PAM pulses.
The second concept, that of eliminating large amplitude PAM pulses and employing gain control, recognizes that when the signal to be transmitted is known to have an amplitude no greater than some given level, then that signal can be increased (by introducing gain) prior to transmission without exceeding the maximum allowable power. At the receiver, the signal is attenuated by a corresponding magnitude. Noise that is added by the transmission medium is thereby also attenuated.
The signal leak concept recognizes that some noise is inevitable, and inserts a fraction of the video frame into the error signal that is transmitted. By appropriate subtraction at the receiver, noise that is unavoidably injected is eventually flushed out of the receiver.
The scrambling concept also deals with the expectation that some noise unavoidably will be accepted by the receiver. The concern is with correlated noise that comes about through fading and multiple path receptions. The effects of such noise are minimized by scrambling the signal at the transmitter's end and unscrambling it at the receiver's end to produce snow-like noise at the receiver when burst errors occur, rather than correlated noise (such as "ghosts").
The above-described techniques deal with each frame signal essentially as an independent entity. No attempt is made to make use of whatever correlation exists between frames. More importantly, no attempt is made to take advantage of correlated components in the noise. Yet, some benefits (from the standpoint of noise immunity) may be derived from such considerations.